1. Field of the Invention
The invention relates to a push-pull power regulator with a circuit protecting the switching transistors of the power regulator against current overload.
In such a power regulator known from German Published Application No. 23 38 538 a resistance of a relatively low value is included in the circuit in series to the load, where a correspondingly high voltage drops in cases of current overload. The voltage drop at the resistance is compared with a reference voltage, and if a predetermined threshold value is reached the feeding voltage of the power regulator is switched off via a relay. This arrangement has various disadvantages. Since in the first place the transistors are to be protected against overcurrent, with the current, however, being measured at the secondary side of the power regulator transformer, the effect achieved is only indirect and delayed. If there is an excessive overload, this can destroy the transistors. The insertion of a series resistor to the load unfavorably increases the internal resistance of the power regulator, and besides the current measuring at the secondary side of the transformer rather than at the primary side can create additional error sources.
Another disadvantage of this known power regulator is that the working range of output current and output voltage is dimensioned for a specific use and is relatively narrow, and that outside this range specific regulator circuits for current or voltage, respectively, have to be provided.
The current overload protection used in this known device furthermore does not come up to actual requirements, since the regulator would switch off also in case of one single short term overload. Such a short term overload does not necessarily represent a risk for the transistors, so the protective circuit should be able to tolerate it.
Other known protective circuits for push-pull power regulators refer to the prevention of simultaneous current conduct through the two transistors, since in such an overlapped conducting time the transformer does not establish a magnetic field, and since consequently the current flowing through the transistors would increase in excess of the permissible maximum limit (IBM Technical Disclosure Bulletin, December, 1981, pp. 3887-3888), and to the prevention of asymmetrical current conduct through the two switching transistors (IBM Technical Disclosure Bulletin, November 1982, pp. 2765-2766).
IBM Technical Disclosure Bulletin, April 1981, pp. 4994-4995) describes a switching regulator with an overload-protected individual transistor. The protective circuit permits the time-limitation of individual pulse-width modulated driver pulses as well as the switching-off of the regulator in case of a continuous overload. The switching-off is based on the characteristic of the control loop in the regulator according to which with a decreasing output voltage the error signal used to control the pulse width modulator is increasing. This error signal is directed via a low-pass filter, and the application of further driver pulses to the switching transistor is prevented when the error signal had been on a high amplitude for a long time. The disadvantage of this kind of using the error signal to switch off the regulator is that it is not the actual risky quantity, i.e. the current through the transistor, which is used to switch off the regulator but an undervoltage at the output of the regulator. The overload protection circuit can act on the individual driver pulses only during each switching cycle. As owing to the charge storage period of the transistor the conductive period of the switching transistor cannot pass beneath a certain limit there is the risk, in spite of the protective circuit, of the transistor being destroyed. Another disadvantage of using the error signal to switch off the regulator is that part of the upper region of this error signal cannot be utilized for conventional control since switching is effected from a specific value, and since a predetermined safety distance to this value has to be observed.
IBM Technical Disclosure Bulletin, October 1981, pp. 2435-2436 also refers to an overcurrent protection in a power regulator with separate switching transistor. In this regulator, too, the current through each individual switching transistor is utilized to limit, if necessary, the pulse width modulated driver pulses. Out of the unlimited pulse width modulated driver pulse, and the possibly limited one, a differential signal is generated which after integration can be used to switch off the control oscillator. On principle, the disadvantages of this circuit are therefore the same as described in connection with the preceding circuit. Another common feature of these circuits is that no devices are provided which permit a re-start after a switching-off of the power regulator, with the negative consequence that the regulator will be switched off for a short-term and possibly unimpairing overload as well as for an overload extending over a longer period.